1. Field of the Invention
The present invention pertains to rotary drill bits employed for coring and drilling well bores in an earth formation. More particularly, the present invention pertains to an improved arrangement and shape of cutter bodies, and attached cutter elements, in which arrangement the cutting elements are oriented to impart a secondary rotation to the cutter body, while each cutter body is simultaneously rotating with a primary rotation around a longitudinal axis of a drill string, cutting away and crushing the earth formation being drilled.
2. Brief Description of the Prior Art
The cutter body employed in prior art rotary drill bits is rotatively connected, in groups of two or more, to a main body structure. Each cutter body is generally of conical or frustoconical-shape, a base of the cutter body positioned at the outside of the drill bit to gauge the size of the hole being drilled. Cutting elements or teeth are attached to or formed on the exterior surface of each cutter body. The cutting elements are rotated about a rotational axis of the cutter body, and in a rotation about a longitudinal axis of a drill string, to engage an earth formation being drilled. The contact of the cutting elements with the earth formation during rotation loosens particulate matter or cuttings for removal by drilling fluids in a conventionally known manner.
In some of the prior art, the rotational axis of each cutter body passes through the longitudinal axis of the drill string, the drill string being rotated to drill the earth formation. Each cutter body rotates in direct proportion to the primary rotation of the drill string about the longitudinal axis. Very little abrasive cutting action bears on the well bore being drilled. The proportional relationship is virtually the same relationship as exists between a wheel and axle, as the drill string rotates, the cutter body rotates. The primary force acting to abrade away and remove the particulate matter is the weight of the drill bit and drill string producing a crushing type of action. Little abrasion is imparted to the formation by the rotation of each cutter body.
It is known in the prior art to move or offset the rotational axis of each cutter body so that the axis does not pass through the longitudinal axis of the drill string. Offsetting the rotational axis of each cutter body increases the abrasion, or scraping, force of the rotary drill bit against the earth formation. The offset cutter body of the prior art has heretofore been small enough that a primary component of the rotation of the cutter body remains proportional to the rotation of the drill string.
An axis of rotation of the cutter body perpendicular to the primary rotational axis and with the rotational end supports of each cutter body equi-distant from the longitudinal axis of the drill string, is seen in a well reamer, U.S. Pat. No. 2,174,587 to D. Love. As used herein, this orientation is defined as fully offset. The well reamer of Love is not a rotary drill bit, but rather is used to enlarge the bore hole formed by a rotary drill bit. Love therefore does not act on the bottom of the bore hole itself but rather only at the periphery thereof, after the hole has been drilled.
The patent to C. Reed, U.S. Pat. No. 1,236,982, shows rotational axes for a pair of cutter bodies perpendicular to the longitudinal axis of a drill stem in a rotary drill bit. The cutter body elements are not fully offset, in that the cutter body end support points are not equi-distant from the longitudinal axis of the drill string. Reed does show spiral teeth or cutter elements that apparently impart a rotation to each cutter body. The imparted rotation of each cutter body remains proportional to the rotation of the drill string. The two cutter bodies of Reed have cutter elements of opposite angular orientation along the cutter body surface so that one cutter body does not track the previously made cutting pattern. Other cutter body cutter elements of spiral tooth orientation are known. Skewed teeth are seen in H. Mitchell, U.S. Pat. No. 4,161,225.
A rotary cutting action as well as a crushing or abrading action is seen in H. Hughes, U.S. Pat. No. 1,124,242. Hughes uses both crushing rollers and cutter bodies to provide both rotary and crushing action. A combination rotary and percussion drill bit in S. Skidmore, U.S. Pat. No. 3,885,638, wherein helical grooves extend the length of the generally frustoconically-shaped bit. Skidmore apparently would be used with a drill string but is not a multiple cutter body rotary bit.
The patent to B. Munson, U.S. Pat. No. 4,408,671 shows variation of taper and angle of the teeth between cutter bodies. Different numbers of teeth between cutter bodies are seen in J. Strauss et al, U.S. Pat. No. 1,045,756, while F. Phelps, U.S. Pat. No. 4,187,922 varies the angle of the various teeth or cutter elements.
Of general interest in rotary drill bits are the patents to R. Evans, U.S. Pat. No. 4,148,368, B. Austin, U.S. Pat. No. 3,468,583, H. Bovenkerk, U.S. Pat. No. 4,109,737 and W. Daniels et al U.S. Pat. No. 4,333,540.